A high quality speech is reproduced with a small data amount in speech coding and decoding for performing compression coding and decoding of a speech signal to a digital signal. In speech coding method according to a code-excited linear prediction (CELP) speech coding, a noise level of a speech in a concerning coding period is evaluated by using a code or coding result of at least one of spectrum information, power information, and pitch information, and various excitation codebooks are used based on an evaluation result.

Claim:

The invention claimed is:

1. A speech decoding method for decoding a speech code including a linear prediction parameter code, an adaptive code, and a gain code according to code-excited linearprediction (CELP), the speech decoding method comprising: decoding a linear prediction parameter from the linear prediction parameter code; obtaining an adaptive code vector corresponding to the adaptive code concerning a decoding period from anadaptive codebook; obtaining a time series vector concerning the decoding period from an excitation codebook; decoding a gain of the adaptive code vector and a gain of an excitation code vector from the gain code; performing a mathematical operationon the time series vector and another operand which is selected on the basis of the gain of the adaptive code vector; obtaining an excitation code vector based on the result of the mathematical operation; weighting the adaptive code vector and theexcitation code vector by using the decoded gains; obtaining an excitation signal by adding the weighted adaptive code vector and the weighted excitation code vector; and synthesizing a speech by using the excitation signal and the linear predictionparameter.

2. A speech decoding apparatus for decoding a speech code including a linear prediction parameter code, an adaptive code, and a gain code according to code-excited linear prediction (CELP), the speech decoding apparatus comprising: a linearprediction parameter decoding unit for decoding a linear prediction parameter from the linear prediction parameter code; an adaptive code vector obtaining unit for obtaining an adaptive code vector corresponding to the adaptive code concerning adecoding period from an adaptive codebook; a time series vector obtaining unit for obtaining a time series vector concerning the decoding period from an excitation codebook; a gain decoding unit for decoding a gain of the adaptive code vector and again of an excitation code vector from the gain code; an excitation code vector obtaining unit for performing a mathematical operation on the time series vector and another operand which is selected on the basis of the gain of the adaptive code vector,and obtaining an excitation code vector based on the result of the mathematical operation; a weighting unit for weighting the adaptive code vector and the excitation code vector by using the decoded gains; an obtaining unit for obtaining an excitationsignal by adding the weighted adaptive code vector and the weighted excitation code vector; and a synthesizing unit for synthesizing a speech by using the excitation signal and the linear prediction parameter.

Description:

BACKGROUND OF THE INVENTION

(1) Field of the Invention

This invention relates to methods for speech coding and decoding and apparatuses for speech coding and decoding for performing compression coding and decoding of a speech signal to a digital signal. Particularly, this invention relates to amethod for speech coding, method for speech decoding, apparatus for speech coding, and apparatus for speech decoding for reproducing a high quality speech at low bit rates.

In CELP speech coding, a speech in a frame of about 5-50 ms is divided into spectrum information and excitation information, and coded.

Explanations are made on operations in the CELP speech coding method. In the encoder 101, the linear prediction parameter analyzing means 105 analyzes an input speech S101, and extracts a linear prediction parameter, which is spectruminformation of the speech. The linear prediction parameter coding means 106 codes the linear prediction parameter, and sets a coded linear prediction parameter as a coefficient for the synthesis filter 107.

Explanations are made on coding of excitation information.

An old excitation signal is stored in the adaptive codebook 108. The adaptive codebook 108 outputs a time series vector, corresponding to an adaptive code inputted by the distance calculator 111, which is generated by repeating the oldexcitation signal periodically.

A plurality of time series vectors trained by reducing distortion between speech for training and its coded speech, for example, is stored in the excitation codebook 109. The excitation codebook 109 outputs a time series vector corresponding toan excitation code inputted by the distance calculator 111.

Each of the time series vectors outputted from the adaptive codebook 108 and excitation codebook 109 is weighted by using a respective gain provided by the gain coding means 110 and added by the weighting-adding means 138. Then, an additionresult is provided to the synthesis filter 107 as excitation signals, and coded speech is produced. The distance calculating means 111 calculates a distance between the coded speech and the input speech S101, and searches an adaptive code, excitationcode, and gains for minimizing the distance. When the above-stated coding is over, a linear prediction parameter code and the adaptive code, excitation code, and gain codes for minimizing a distortion between the input speech and the coded speech areoutputted as a coding result.

Explanations are made on operations in the CELP speech decoding method.

In the decoder 102, the linear prediction parameter decoding means 112 decodes the linear prediction parameter code to the linear prediction parameter, and sets the linear prediction parameter as a coefficient for the synthesis filter 113. Theadaptive codebook 114 outputs a time series vector corresponding to an adaptive code, which is generated by repeating an old excitation signal periodically. The excitation codebook 115 outputs a time series vector corresponding to an excitation code. The time series vectors are weighted by using respective gains, which are decoded from the gain codes by the gain decoding means 116, and added by the weighting-adding means 139. An addition result is provided to the synthesis filter 113 as anexcitation signal, and an output speech S103 is produced.

Among the CELP speech coding and decoding method, an improved speech coding and decoding method for reproducing a high quality speech according to the related art is described in "Phonetically--based vector excitation coding of speech at 3.6kbps," ICASSP '89, pp. 49-52, by S. Wang and A. Gersho in 1989.

FIG. 7 shows an example of a whole configuration of the speech coding and decoding method according to the related art, and same signs are used for means corresponding to the means in FIG. 6.

In FIG. 7, the encoder 101 includes a speech state deciding means 117, excitation codebook switching means 118, first excitation codebook 119, and second excitation codebook 120. The decoder 102 includes an excitation codebook switching means121, first excitation codebook 122, and second excitation codebook 123.

Explanations are made on operations in the coding and decoding method in this configuration. In the encoder 101, the speech state deciding means 117 analyzes the input speech S101, and decides a state of the speech is which one of two states,e.g., voiced or unvoiced. The excitation codebook switching means 118 switches the excitation codebooks to be used in coding based on a speech state deciding result. For example, if the speech is voiced, the first excitation codebook 119 is used, andif the speech is unvoiced, the second excitation codebook 120 is used. Then, the excitation codebook switching means 118 codes which excitation codebook is used in coding.

In the decoder 102, the excitation codebook switching means 121 switches the first excitation codebook 122 and the second excitation codebook 123 based on a code showing which excitation codebook was used in the encoder 101, so that theexcitation codebook, which was used in the encoder 101, is used in the decoder 102. According to this configuration, excitation codebooks suitable for coding in various speech states are provided, and the excitation codebooks are switched based on astate of an input speech. Hence, a high quality speech can be reproduced.

A speech coding and decoding method of switching a plurality of excitation codebooks without increasing a transmission bit number according to the related art is disclosed in Japanese Unexamined Published Patent Application 8-185198. Theplurality of excitation codebooks is switched based on a pitch frequency selected in an adaptive codebook, and an excitation codebook suitable for characteristics of an input speech can be used without increasing transmission data.

As stated, in the speech coding and decoding method illustrated in FIG. 6 according to the related art, a single excitation codebook is used to produce a synthetic speech. Non-noise time series vectors with many pulses should be stored in theexcitation codebook to produce a high quality coded speech even at low bit rates. Therefore, when a noise speech, e.g., background noise, fricative consonant, etc., is coded and synthesized, there is a problem that a coded speech produces an unnaturalsound, e.g., "Jiri-Jiri" and "Chiri-Chiri." This problem can be solved, if the excitation codebook includes only noise time series vectors. However, in that case, a quality of the coded speech degrades as a whole.

In the improved speech coding and decoding method illustrated in FIG. 7 according to the related art, the plurality of excitation codebooks is switched based on the state of the input speech for producing a coded speech. Therefore, it ispossible to use an excitation codebook including noise time series vectors in an unvoiced noise period of the input speech and an excitation codebook including non-noise time series vectors in a voiced period other than the unvoiced noise period, forexample. Hence, even if a noise speech is coded and synthesized, an unnatural sound, e.g., "Jiri-Jiri," is not produced. However, since the excitation codebook used in coding is also used in decoding, it becomes necessary to code and transmit datawhich excitation codebook was used. It becomes an obstacle for lowing bit rates.

According to the speech coding and decoding method of switching the plurality of excitation codebooks without increasing a transmission bit number according to the related art, the excitation codebooks are switched based on a pitch periodselected in the adaptive codebook. However, the pitch period selected in the adaptive codebook differs from an actual pitch period of a speech, and it is impossible to decide if a state of an input speech is noise or non-noise only from a value of thepitch period. Therefore, the problem that the coded speech in the noise period of the speech is unnatural cannot be solved.

This invention was intended to solve the above-stated problems. Particularly, this invention aims at providing speech coding and decoding methods and apparatuses for reproducing a high quality speech even at low bit rates.

BRIEF SUMMARY OF THE INVENTION

In order to solve the above-stated problems, in a speech decoding method according to this invention, a speech code including a linear prediction parameter code, an adaptive code, and a gain code is received. A linear prediction parameter isdecoded from the linear prediction parameter code, and a gain of an adaptive code vector and a gain of an excitation code vector is decoded from the gain code. An adaptive code vector is obtained corresponding to the adaptive code, and a time seriesvector is obtained from an excitation codebook. A mathematical operation is performed on the time series vector and another operand, the other operand being selected on the basis of the gain of the adaptive code vector. An excitation code vector isobtained based on the result of the mathematical operation. The adaptive code vector and the excitation code vector are weighted by using the decoded gains. An excitation signal is obtained by adding the weighted adaptive code vector and the weightedexcitation code vector, and a speech is synthesized by using the excitation signal and the linear prediction parameter.

A speech decoding apparatus according to another invention includes a linear prediction parameter decoder for decoding a linear prediction parameter code to a linear prediction parameter, a gain decoder for decoding a gain code into a gain for anadaptive code vector and a gain for an excitation code vector, an excitation codebook storing a plurality of time series vectors, an excitation code vector obtainer for obtaining an excitation code vector by performing a mathematical operation on a timeseries vector obtained from the excitation codebook and another operand selected on the basis of the gain of the adaptive code vector, a weighting-adder for obtaining an excitation signal by weighting the adaptive code vector and the excitation codevector depending on their respective decoded gains and adding them together, and a synthesis filter for producing a decoded speech based on the excitation signal and the decoded linear prediction parameter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a block diagram of a whole configuration of a speech coding and speech decoding apparatus in embodiment 1 of this invention;

FIG. 2 shows a table for explaining an evaluation of a noise level in embodiment 1 of this invention illustrated in FIG. 1;

FIG. 3 shows a block diagram of a whole configuration of a speech coding and speech decoding apparatus in embodiment 3 of this invention;

FIG. 4 shows a block diagram of a whole configuration of a speech coding and speech decoding apparatus in embodiment 5 of this invention;

FIG. 5 shows a schematic line chart for explaining a decision process of weighting in embodiment 5 illustrated in FIG. 4;

FIG. 6 shows a block diagram of a whole configuration of a CELP speech coding and decoding apparatus according to the related art;

FIG. 7 shows a block diagram of a whole configuration of an improved CELP speech coding and decoding apparatus according to the related art; and

FIG. 8 shows a block diagram of a whole configuration of a speech coding and decoding apparatus according to embodiment 8 of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Explanations are made on embodiments of this invention with reference to drawings.

In the encoder 1, the linear prediction parameter analyzer 5 analyzes the input speech S1, and extracts a linear prediction parameter, which is spectrum information of the speech. The linear prediction parameter encoder 6 codes the linearprediction parameter. Then, the linear prediction parameter encoder 6 sets a coded linear prediction parameter as a coefficient for the synthesis filter 7, and also outputs the coded linear prediction parameter to the noise level evaluator 24.

Explanations are made on coding of excitation information.

An old excitation signal is stored in the adaptive codebook 8, and a time series vector corresponding to an adaptive code inputted by the distance calculator 11, which is generated by repeating an old excitation signal periodically, is outputted. The noise level evaluator 24 evaluates a noise level in a concerning coding period based on the coded linear prediction parameter inputted by the linear prediction parameter encoder 6 and the adaptive code, e.g., a spectrum gradient, short-termprediction gain, and pitch fluctuation as shown in FIG. 2, and outputs an evaluation result to the excitation codebook switch 25. The excitation codebook switch 25 switches excitation codebooks for coding based on the evaluation result of the noiselevel. For example, if the noise level is low, the first excitation codebook 19 is used, and if the noise level is high, the second excitation codebook 20 is used.

The first excitation codebook 19 stores a plurality of non-noise time series vectors, e.g., a plurality of time series vectors trained by reducing a distortion between a speech for training and its coded speech. The second excitation codebook 20stores a plurality of noise time series vectors, e.g., a plurality of time series vectors generated from random noises. Each of the first excitation codebook 19 and the second excitation codebook 20 outputs a time series vector respectivelycorresponding to an excitation code inputted by the distance calculator 11. Each of the time series vectors from the adaptive codebook 8 and one of first excitation codebook 19 or second excitation codebook 20 are weighted by using a respective gainprovided by the gain encoder 10, and added by the weighting-adder 38. An addition result is provided to the synthesis filter 7 as excitation signals, and a coded speech is produced. The distance calculator 11 calculates a distance between the codedspeech and the input speech S1, and searches an adaptive code, excitation code, and gain for minimizing the distance. When this coding is over, the linear prediction parameter code and an adaptive code, excitation code, and gain code for minimizing thedistortion between the input speech and the coded speech are outputted as a coding result S2. These are characteristic operations in the speech coding method in embodiment 1.

Explanations are made on the decoder 2. In the decoder 2, the linear prediction parameter decoder 12 decodes the linear prediction parameter code to the linear prediction parameter, and sets the decoded linear prediction parameter as acoefficient for the synthesis filter 13, and outputs the decoded linear prediction parameter to the noise level evaluator 26.

Explanations are made on decoding of excitation information. The adaptive codebook 14 outputs a time series vector corresponding to an adaptive code, which is generated by repeating an old excitation signal periodically. The noise levelevaluator 26 evaluates a noise level by using the decoded linear prediction parameter inputted by the linear prediction parameter decoder 12 and the adaptive code in a same method with the noise level evaluator 24 in the encoder 1, and outputs anevaluation result to the excitation codebook switch 27. The excitation codebook switch 27 switches the first excitation codebook 22 and the second excitation codebook 23 based on the evaluation result of the noise level in a same method with theexcitation codebook switch 25 in the encoder 1.

A plurality of non-noise time series vectors, e.g., a plurality of time series vectors generated by training for reducing a distortion between a speech for training and its coded speech, is stored in the first excitation codebook 22. A pluralityof noise time series vectors, e.g., a plurality of vectors generated from random noises, is stored in the second excitation codebook 23. Each of the first and second excitation codebooks outputs a time series vector respectively corresponding to anexcitation code. The time series vectors from the adaptive codebook 14 and one of first excitation codebook 22 or second excitation codebook 23 are weighted by using respective gains, decoded from gain codes by the gain decoder 16, and added by theweighting-adder 39. An addition result is provided to the synthesis filter 13 as an excitation signal, and an output speech S3 is produced. These are operations are characteristic operations in the speech decoding method in embodiment 1.

In embodiment 1, the noise level of the input speech is evaluated by using the code and coding result, and various excitation codebooks are used based on the evaluation result. Therefore, a high quality speech can be reproduced with a small dataamount.

In embodiment 1, the plurality of time series vectors is stored in each of the excitation codebooks 19, 20, 22, and 23. However, this embodiment can be realized as far as at least a time series vector is stored in each of the excitationcodebooks.

Embodiment 2

In embodiment 1, two excitation codebooks are switched. However, it is also possible that three or more excitation codebooks are provided and switched based on a noise level.

In embodiment 2, a suitable excitation codebook can be used even for a medium speech, e.g., slightly noisy, in addition to two kinds of speech, i.e., noise and non-noise. Therefore, a high quality speech can be reproduced.

Embodiment 3

FIG. 3 shows a whole configuration of a speech coding method and speech decoding method in embodiment 3 of this invention. In FIG. 3, same signs are used for units corresponding to the units in FIG. 1. In FIG. 3, excitation codebooks 28 and 30store noise time series vectors, and samplers 29 and 31 set an amplitude value of a sample with a low amplitude in the time series vectors to zero.

Operations are explained. In the encoder 1, the linear prediction parameter analyzer 5 analyzes the input speech S1, and extracts a linear prediction parameter, which is spectrum information of the speech. The linear prediction parameterencoder 6 codes the linear prediction parameter. Then, the linear prediction parameter encoder 6 sets a coded linear prediction parameter as a coefficient for the synthesis filter 7, and also outputs the coded linear prediction parameter to the noiselevel evaluator 24.

Explanations are made on coding of excitation information. An old excitation signal is stored in the adaptive codebook 8, and a time series vector corresponding to an adaptive code inputted by the distance calculator 11, which is generated byrepeating an old excitation signal periodically, is outputted. The noise level evaluator 24 evaluates a noise level in a concerning coding period by using the coded linear prediction parameter, which is inputted from the linear prediction parameterencoder 6, and an adaptive code, e.g., a spectrum gradient, short-term prediction gain, and pitch fluctuation, and outputs an evaluation result to the sampler 29.

The excitation codebook 28 stores a plurality of time series vectors generated from random noises, for example, and outputs a time series vector corresponding to an excitation code inputted by the distance calculator 11. If the noise level islow in the evaluation result of the noise, the sampler 29 outputs a time series vector, in which an amplitude of a sample with an amplitude below a determined value in the time series vectors, inputted from the excitation codebook 28, is set to zero, forexample. If the noise level is high, the sampler 29 outputs the time series vector inputted from the excitation codebook 28 without modification. Each of the times series vectors from the adaptive codebook 8 and the sampler 29 is weighted by using arespective gain provided by the gain encoder 10 and added by the weighting-adder 38. An addition result is provided to the synthesis filter 7 as excitation signals, and a coded speech is produced. The distance calculator 11 calculates a distancebetween the coded speech and the input speech S1, and searches an adaptive code, excitation code, and gain for minimizing the distance. When coding is over, the linear prediction parameter code and the adaptive code, excitation code, and gain code forminimizing a distortion between the input speech and the coded speech are outputted as a coding result S2. These are characteristic operations in the speech coding method in embodiment 3.

Explanations are made on the decoder 2. In the decoder 2, the linear prediction parameter decoder 12 decodes the linear prediction parameter code to the linear prediction parameter. The linear prediction parameter decoder 12 sets the linearprediction parameter as a coefficient for the synthesis filter 13, and also outputs the linear prediction parameter to the noise level evaluator 26.

Explanations are made on decoding of excitation information. The adaptive codebook 14 outputs a time series vector corresponding to an adaptive code, generated by repeating an old excitation signal periodically. The noise level evaluator 26evaluates a noise level by using the decoded linear prediction parameter inputted from the linear prediction parameter decoder 12 and the adaptive code in a same method with the noise level evaluator 24 in the encoder 1, and outputs an evaluation resultto the sampler 31.

The excitation codebook 30 outputs a time series vector corresponding to an excitation code. The sampler 31 outputs a time series vector based on the evaluation result of the noise level in same processing with the sampler 29 in the encoder 1. Each of the time series vectors outputted from the adaptive codebook 14 and sampler 31 are weighted by using a respective gain provided by the gain decoder 16, and added by the weighting-adder 39. An addition result is provided to the synthesis filter13 as an excitation signal, and an output speech S3 is produced.

In embodiment 3, the excitation codebook storing noise time series vectors is provided, and an excitation with a low noise level can be generated by sampling excitation signal samples based on an evaluation result of the noise level the speech. Hence, a high quality speech can be reproduced with a small data amount. Further, since it is not necessary to provide a plurality of excitation codebooks, a memory amount for storing the excitation codebook can be reduced.

Embodiment 4

In embodiment 3, the samples in the time series vectors are either sampled or not. However, it is also possible to change a threshold value of an amplitude for sampling the samples based on the noise level. In embodiment 4, a suitable timeseries vector can be generated and used also for a medium speech, e.g., slightly noisy, in addition to the two types of speech, i.e., noise and non-noise. Therefore, a high quality speech can be reproduced.

Embodiment 5

FIG. 4 shows a whole configuration of a speech coding method and a speech decoding method in embodiment 5 of this invention, and same signs are used for units corresponding to the units in FIG. 1.

In FIG. 4, first excitation codebooks 32 and 35 store noise time series vectors, and second excitation codebooks 33 and 36 store non-noise time series vectors. The weight determiners 34 and 37 are also illustrated.

Operations are explained. In the encoder 1, the linear prediction parameter analyzer 5 analyzes the input speech S1, and extracts a linear prediction parameter, which is spectrum information of the speech. The linear prediction parameterencoder 6 codes the linear prediction parameter. Then, the linear prediction parameter encoder 6 sets a coded linear prediction parameter as a coefficient for the synthesis filter 7, and also outputs the coded prediction parameter to the noise levelevaluator 24.

Explanations are made on coding of excitation information. The adaptive codebook 8 stores an old excitation signal, and outputs a time series vector corresponding to an adaptive code inputted by the distance calculator 11, which is generated byrepeating an old excitation signal periodically. The noise level evaluator 24 evaluates a noise level in a concerning coding period by using the coded linear prediction parameter, which is inputted from the linear prediction parameter encoder 6 and theadaptive code, e.g., a spectrum gradient, short-term prediction gain, and pitch fluctuation, and outputs an evaluation result to the weight determiner 34.

The first excitation codebook 32 stores a plurality of noise time series vectors generated from random noises, for example, and outputs a time series vector corresponding to an excitation code. The second excitation codebook 33 stores aplurality of time series vectors generated by training for reducing a distortion between a speech for training and its coded speech, and outputs a time series vector corresponding to an excitation code inputted by the distance calculator 11. The weightdeterminer 34 determines a weight provided to the time series vector from the first excitation codebook 32 and the time series vector from the second excitation codebook 33 based on the evaluation result of the noise level inputted from the noise levelevaluator 24, as illustrated in FIG. 5, for example. Each of the time series vectors from the first excitation codebook 32 and the second excitation codebook 33 is weighted by using the weight provided by the weight determiner 34, and added. The timeseries vector outputted from the adaptive codebook 8 and the time series vector, which is generated by being weighted and added, are weighted by using respective gains provided by the gain encoder 10, and added by the weighting-adder 38. Then, anaddition result is provided to the synthesis filter 7 as excitation signals, and a coded speech is produced. The distance calculator 11 calculates a distance between the coded speech and the input speech S1, and searches an adaptive code, excitationcode, and gain for minimizing the distance. When coding is over, the linear prediction parameter code, adaptive code, excitation code, and gain code for minimizing a distortion between the input speech and the coded speech, are outputted as a codingresult.

Explanations are made on the decoder 2. In the decoder 2, the linear prediction parameter decoder 12 decodes the linear prediction parameter code to the linear prediction parameter. Then, the linear prediction parameter decoder 12 sets thelinear prediction parameter as a coefficient for the synthesis filter 13, and also outputs the linear prediction parameter to the noise evaluator 26.

Explanations are made on decoding of excitation information. The adaptive codebook 14 outputs a time series vector corresponding to an adaptive code by repeating an old excitation signal periodically. The noise level evaluator 26 evaluates anoise level by using the decoded linear prediction parameter, which is inputted from the linear prediction parameter decoder 12, and the adaptive code in a same method with the noise level evaluator 24 in the encoder 1, and outputs an evaluation resultto the weight determiner 37.

The first excitation codebook 35 and the second excitation codebook 36 output time series vectors corresponding to excitation codes. The weight determiner 37 weights based on the noise level evaluation result inputted from the noise levelevaluator 26 in a same method with the weight determiner 34 in the encoder 1. Each of the time series vectors from the first excitation codebook 35 and the second excitation codebook 36 is weighted by using a respective weight provided by the weightdeterminer 37, and added. The time series vector outputted from the adaptive codebook 14 and the time series vector, which is generated by being weighted and added, are weighted by using respective gains decoded from the gain codes by the gain decoder16, and added by the weighting-adder 39. Then, an addition result is provided to the synthesis filter 13 as an excitation signal, and an output speech S3 is produced.

In embodiment 5, the noise level of the speech is evaluated by using a code and coding result, and the noise time series vector or non-noise time series vector are weighted based on the evaluation result, and added. Therefore, a high qualityspeech can be reproduced with a small data amount.

Embodiment 6

In embodiments 1-5, it is also possible to change gain codebooks based on the evaluation result of the noise level. In embodiment 6, a most suitable gain codebook can be used based on the excitation codebook. Therefore, a high quality speechcan be reproduced.

Embodiment 7

In embodiments 1-6, the noise level of the speech is evaluated, and the excitation codebooks are switched based on the evaluation result. However, it is also possible to decide and evaluate each of a voiced onset, plosive consonant, etc., andswitch the excitation codebooks based on an evaluation result. In embodiment 7, in addition to the noise state of the speech, the speech is classified in more details, e.g., voiced onset, plosive consonant, etc., and a suitable excitation codebook canbe used for each state. Therefore, a high quality speech can be reproduced.

Embodiment 8

In embodiments 1-6, the noise level in the coding period is evaluated by using a spectrum gradient, short-term prediction gain, pitch fluctuation. However, it is also possible to evaluate the noise level by using a ratio of a gain value againstan output from the adaptive codebook as illustrated in FIG. 8, in which similar elements are labeled with the same reference numerals.

INDUSTRIAL APPLICABILITY

In the speech coding method, speech decoding method, speech coding apparatus, and speech decoding apparatus according to this invention, a noise level of a speech in a concerning coding period is evaluated by using a code or coding result of atleast one of the spectrum information, power information, and pitch information, and various excitation codebooks are used based on the evaluation result. Therefore, a high quality speech can be reproduced with a small data amount.

In the speech coding method and speech decoding method according to this invention, a plurality of excitation codebooks storing excitations with various noise levels is provided, and the plurality of excitation codebooks is switched based on theevaluation result of the noise level of the speech. Therefore, a high quality speech can be reproduced with a small data amount.

In the speech coding method and speech decoding method according to this invention, the noise levels of the time series vectors stored in the excitation codebooks are changed based on the evaluation result of the noise level of the speech. Therefore, a high quality speech can be reproduced with a small data amount.

In the speech coding method and speech decoding method according to this invention, an excitation codebook storing noise time series vectors is provided, and a time series vector with a low noise level is generated by sampling signal samples inthe time series vectors based on the evaluation result of the noise level of the speech. Therefore, a high quality speech can be reproduced with a small data amount.

In the speech coding method and speech decoding method according to this invention, the first excitation codebook storing noise time series vectors and the second excitation codebook storing non-noise time series vectors are provided, and thetime series vector in the first excitation codebook or the time series vector in the second excitation codebook is weighted based on the evaluation result of the noise level of the speech, and added to generate a time series vector. Therefore, a highquality speech can be reproduced with a small data amount.